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Full text of "Handbook Of Chemical Engineering - I"

PYROMETRY
453
Actually it is at about the same temperature as the less bright surrounding wall. On account of reflection a corresponding bright patch appears on the opposite wall although this wall may be free from coke. It is evident that the measurement of temperature of a portion of a non-uniformly heated furnace by means of an optical pyrometer is difficult unless the precautions suggested above are taken. As soon as the furnace attains temperature uniformity and equilibrium the optical pyrometer gives the true temperature very easily and readily.
When an optical pyrometer is sighted on a glowing material in the open it reads too low. Certain materials, important industrially, have a very high emissivity so that the corrections necessary to add to the observed temperatures to convert them into true temperatures are small. Thus with iron oxide the correction is only 10° at 1,200°C. The corrections are very large for clear molten metals, but are smaller for the oxides which soon form on the molten surface when exposed to the air. Table 12 shows the true temperatures corresponding to the temperatures observed when sighting on certain materials in the open. For temperature control it is unnecessary to apply these corrections. The observed temperatures, although known to be low, will be low by the same amount from time to time and hence will serve just as satisfac-
TABLE  12.—TRUE TEMPERATURES  vs.  APPARENT  TEMPERATURES  MEASURED BY
OPTICAL PYROMETERS USING RED LIGHT (X = 0.65ju) WHEN SIGHTED UPON
THE FOLLOWING MATERIALS IN THE OPEN
Observed temperature, degrees Centigrade	True temperature, degrees Centigrade						
	Molten copper	Molten iron1	Solid iron oxide	Solid nickel oxide	Nichrome or chromel	Molten slag2	Bright platinum
700 800 900 950 1,000 1,050 1,100 1,150 1,200 1,250 1,300 1,400 1,500 1,600 1,700 1,750	1,088 1,150 1,213 1,277 1,341 1,405 1,470 1,536	1,183 1,239 1,296 1,353 1,410 1,525 1,641 1,758 1,876 1,935	700 801 902 953 1,004 1,055 1,106 1,158 1,210	701 802 904 955 1,007 1,058 1,110 1,162 1,215 1,267 1,320	702 804 906 958 1,010 1,063 1,116 1,170 1,224		750 861 973 1,030 1,087 1,144 1,202 1,260 1,320 1,3*75 1,435 1,555 1,675
							
						.....	
							
						1,455 1,565 1,670 1,780 1,830	
							
							
							
1 Computed for E\
2 Computed for E\
0.40, this being the best value for ordinary steel practice. • 0.65, an average value for liquid slags.
torily for reproducing temperature conditions in any process as the corrected temperatures. The above statement must be modified if factors other than emissivity of the material require consideration. For example, reproducible results cannot be